Japanese Utility Model Publication SHO 62-102110 discloses an apparatus for measuring the radius of a tire tread. This comprises a means for measuring the crown radius of a tire which is an optical sensor for measuring the distance to the crown portion of the tire from the sensor without contact to the tire. The sensor is mounted so that it can be moved in the direction of the axis of the tire and thus scan across the tread region. The output signals in relation to the distance from the sensor to the surface of the tire and the position of the optical sensor in relation to the axial direction of the tire are both used to calculate the profile of the tire crown portion by means of measuring three points on the tread profile and calculating from the said three points.
Thus, the conventional apparatus for measuring the radius of the crown of a tire is adapted to determine the crown radius from three points on the tread profile and is therefore unable to accurately and rapidly measure the profile of the tread shoulder portions. Furthermore the apparatus fails to give measurements of sufficiently high accuracy since it does not measure variations in crown shape and simply averages over three points.